The latest celebrity to make the switch to a vegan diet is US talk-show host Rosie O'Donnell, who suffered a potentially fatal heart attack on August 14.

O'Donnell was admitted to hospital and doctors inserted a stent to clear her coronary artery, which was 99 per cent clogged. Almost immediately, 50-year-old O'Donnell cut all animal products from her diet and nine days later tweeted: "nine pounds lost – eating a plant based diet #likebillclinton."

O'Donnell also spoke to Dr Caldwell Esselstyn, author of Prevent and Reverse Heart Disease, in which the former surgeon and Olympic rowing champion argues that a vegan diet can reverse cardiovascular disease.

Former US president Bill Clinton read Esselstyn's book before switching to a plant-based diet following his quadruple bypass surgery.

"I had been playing Russian roulette," says Clinton, who now consumes no meat, no dairy and no eggs.

"I like the vegetables, the fruits, the beans, the stuff I eat now," Clinton told CNN. "All my blood tests are good, my vital signs are good and I also have, believe it or not, more energy."

Seven-time Grand Slam tennis champion Venus Williams switched to a raw vegan diet last year after being diagnosed with Sjogren's syndrome – an autoimmune disease which left her with fatigue so overwhelming that "sitting in a chair was a huge effort." Williams told CBS, " I've made huge improvements since I was first diagnosed ... changing my diet has made a big difference."

Comedian Russell Brand, a longtime vegetarian, became a vegan last year after watching Forks over Knives - a documentary about the health effects of eating animal-based and processed foods.

One of the more unlikely converts to veganism is former world heavyweight boxing champion, "Iron" Mike Tyson - notorious for biting off part of the ear of rival Evander Holyfield during a 1997 title fight. Tyson turned vegan in 2009 and last year told Fox News that he feels "awesome, incredible."

"When you find out about all the garbage you've been eating ... no wonder I was crazy all those years," he says. "The drugs didn't help either," Tyson jokes.

Others including actor Robin Williams and musician Ozzy Osbourne also switched to a vegan diet because of health concerns, but many celebrities cite moral and ethical grounds for their veganism.

(Ethical vegans avoid all animal products including honey, gelatine and rennet. They also avoid leather, wool and other animal by-products.)

"Around 450 billion animals are factory farmed on our planet every year," says talk show host Ellen DeGeneres. "Dairy cows are kept constantly pregnant to produce milk, while their calves, considered by-products, are put to death almost instantly.

"Animal farming is the number one cause of climate change in the world and has a 40 per cent larger carbon footprint than all global transportation – every car, truck, bus, train and plane combined," says DeGeneres.

Spiderman star Tobey Maguire created headlines in Sydney last year when he sent back a new Mercedes he was given for use during the filming of Baz Luhrmann's The Great Gatsby because he objected to the car's leather seats.

Academy Award nominee Alec Baldwin (30 Rock, The Departed) this week is the public face of a new People for the Ethical Treatment of Animals (PETA) campaign calling for a boycott of circuses that use animals.

Fellow Oscar nominee Joaquin Phoenix (Walk the Line, Gladiator), who became a vegan after a fishing trip as a 3-year-old, narrated the confronting 2005 documentary Earthlings, which examines the suffering of animals for food, fashion, entertainment and medical research.

"Of all the films I have ever made," says Phoenix, "this is the one that gets people talking the most."

"Chickens, cows and pigs in factory farms spend their whole lives in filthy, cramped conditions, only to die a prolonged and painful death," says Casey Affleck (The Assassination of Jesse James by the Coward Robert Ford, Ocean's 11, 12 and 13) in a PETA public service announcement that was banned by US television networks.

"Our inability to communicate with each other and everything that's happening in the world is all a symptom of our greater inability to deal with nature appropriately," says Academy Award nominee Woody Harrelson (The People vs. Larry Flynt), a vegan for more than 20 years.

Cricket legend Greg Chappell, one of the first high-profile Australians to embrace a vegan diet, told Vegan Voice magazine in 2001 that "while the myth of dairy being a 'health food' and meat being necessary for nutrients is allowed to be foisted on an unsuspecting public, most people will continue to ignore the impact that their eating habits have on their health."

Lynda Stoner, star of 80s TV drama Cop Shop and current CEO of Animal Liberation NSW, says she "threw out all my leather products, makeup that had been tested on animals and never ate animal or fish flesh again," after reading Australian philosopher Peter Singer's groundbreaking 1975 book, Animal Liberation.

"Despite being a vegetarian for decades I had always, somewhat bizarrely, kept my head in the sand about the suffering of cows and calves and the trillions of male chicks gassed, macerated or just left to die simply because they had no financial value," says Stoner.

Hollywood's "favourite grandma", 90-year-old Betty White (The Golden Girls, Hot in Cleveland), is a long-time vegan activist who continues to campaign on behalf of several animal rights organisations.

The 20-time Emmy nominee admits she prefers animals to people. "They never lie to you. You know that when they tell you something they mean it. They just love you, it's that simple."